Method and measuring device for examining a compressible tissue

ABSTRACT

A measuring method and a measuring device for examining a compressible tissue, such as articular cartilage, said measuring device comprising an elongated rigid frame ( 1 ), a measuring arm ( 2 ) attached to the frame and preferably comprising a contact surface ( 3 ) to be placed against the tissue to be examined, and a measuring stud ( 4 ) and means ( 5 ) for processing signals obtained from the measuring stud. The measuring stud is an ultrasound probe for emitting ultrasound into the tissue to be examined and receiving ultrasound from the tissue to be examined.

The present invention relates to a method as defined in the preambles ofclaims 1 and 6 and to a measuring device as defined in the preamble ofclaim 8 for examining a compressible tissue, such as articularcartilage.

Articular cartilage consists of differentiated connective tissue thatcontains no blood vessels, lymphatic vessels or nerves. Articularcartilage is the stiffest kind of soft tissue in the human body, yet itis clearly softer than bone. The thickness of articular cartilage variesin different joints from a few micrometers to several millimeters. Thethickness of articular cartilage may also be different in differentparts of joint surfaces. Articular cartilage has two mainfunctions—reducing the stress applied to the subchondral bone andreducing friction in the joint.

The properties of articular cartilage, including mechanical properties,undergo changes during articular cartilage diseases such as arthrosis,rheumatoid arthritis and chondromalacia. Softening of cartilage is oftenthe first perceptible symptom of cartilage decay. On the other hand,e.g. after a corrective treatment of an articular cartilage injury bycellular transfer, the corrective tissue will gradually solidify,allowing measurement of mechanical properties of the tissue to be usedfor the estimation of the results of surgery.

The simplest method used for estimating the stiffness of articularcartilage is to feel the cartilage surface by pressing on it with ametallic instrument in connection with an operation. However, theresults of such measurement are highly subjective and inconsistent.

Various measuring devices have been developed for use in connection withsurgical operations. Prior-art technology in the field of the inventionis primarily represented by patent FI 90616. A tissue stiffnessmeasuring device disclosed in this specification comprises an elongatedrigid frame comprising a contact surface which, in connection with ameasurement, is pressed against the surface of the tissue beingexamined. The frame supports a measuring arm provided with a projectingmeasuring stud or an equivalent, relatively small pin. The measuring armis also provided with a sensor for the measurement of the stress appliedto it via the measuring stud from the tissue being measured.

When this measuring device is used, its contact surface is pressedagainst the tissue to be measured so that the measuring stud projectingoutside the contact surface compresses the cartilage under it. Thus, thecartilage exerts a pressure proportional to its stiffness on themeasuring stud and the measuring arm. This pressure produces measurablestresses in the measuring arm. Corresponding measurable stresses areproduced in the frame when the contact surface is pressed against thecartilage to be measured.

This prior-art measuring device has proved to be a very workable andeffective tool in the examination of compressible tissues, such asarticular cartilage. The main problem with this prior-art device is onlythe scantness of information produced by it. The device was so designedthat the effect of cartilage thickness could be minimized, because thedevice could not be used for the measurement of cartilage thickness. Ininvestigations concerning cartilage tissue, there is clearly a need toobtain other information about the tissue in addition to its stiffness,which does not tell all about the tissue being examined. Thus, a needhas arisen to develop a measuring device that can be used to accomplish,in addition to arthroscopic mechanical stiffness measurement of thetissue, a more detailed characterization of the tissue structure.

As for the features characteristic of the invention, reference is madeto the claims.

In the method of the invention, when a stiff tissue is examined viaarthroscopic indentation measurement, the compression of the tissue isnot effected using a prior-art measuring stud but by means of anultrasound probe, the thickness and compression of the tissue beingmeasured by ultrasound while the compressive force is measured by aknown technique using a strain gauge.

Thus, in the method of the invention, it is possible to determine theabsolute dynamic or static Young's modulus and the modulus of shear.Young's modulus describes the elastic stiffness of articular cartilageunder stress and is therefore a good indicator of mechanicalfunctionability. Like Young's modulus, the modulus of shear is also aneffective indicator of mechanical functionability.

In addition, the method can be used to determine the relaxation speed,i.e. reduction of dynamic stiffness of a tissue and its indentationstiffness dependent on duration of stress as well as other possiblequantitative parameters descriptive of the dynamic behavior of articularcartilage. These parameters take into account the relaxation behaviortypical of articular cartilage and they are therefore indicators of themechanical properties of cartilage under long-time stress. They havealso been found to be predictive of changes in cartilage composition,especially in proteoglycane content, which is an important biochemicalcomponent of cartilage.

In another method according to the invention, to determine structuralproperties of the tissue, an ultrasound probe is held at a distance fromthe tissue surface, preferably inside the frame or arm of the measuringdevice so that there remains between the ultrasound probe and the tissuesurface some physiologic salt solution, with which the joint to beexamined has been filled in connection with arthroscopy. In this way,using an ultrasound probe, pulse-echo measurements can be made on thetissue surface, interior portions of the tissue and/or on the bone underthe tissue.

During measurement, the ultrasound probe can be kept stationary,producing result data about a given point of the tissue. However, themeasurements are performed by moving the ultrasound probe, i.e. byscanning a certain tissue surface with ultrasound.

By examining e.g. articular cartilage in this manner, it is possible todetermine the thickness of the cartilage layer, the ability of thecartilage surface, internal structures and subchondral bone to reflector scatter the sound as well as the attenuation of ultrasound as afunction of frequency.

Of the above-mentioned acoustic parameters:

-   -   Cartilage layer thickness is understandably an important        parameter in an estimation of the condition of cartilage.    -   The ability of cartilage surface and internal structures of        cartilage to reflect/scatter ultrasound are a sensitive        indication of cartilage quality and especially of the condition        of collagenous fibers on cartilage surface.    -   There is no wide experience about the application of        frequency-dependent attenuation to the estimation of cartilage        condition, but this parameter (BUA) has proved to be useful in        the estimation of bone quality. It may be useful in the        estimation of cartilage quality as well.    -   Back-scattering parameters have been successfully applied in the        estimation of both cartilage and bone quality.    -   The ability of subchondral bone to reflect/scatter ultrasound is        a sensitive indication of bone quality.

The device of the invention for the measurement of compressible tissue,such as articular cartilage, comprises an elongated rigid frame and ameasuring arm supported by the frame. Th arm can additionally beprovided with a contact surface to be placed against the tissue to beexamined. The device further comprises a measuring stud and means forprocessing signals obtained from the measuring stud. According to theinvention, the measuring stud is an ultrasound probe for emittingultrasound into the tissue to be examined and for receiving ultrasoundfrom the tissue to be examined.

In an embodiment of the invention, the measuring stud is an elementprojecting from the measuring arm. The measuring arm preferablycomprises a sensor for measuring the force applied to the measuring armfrom the tissue to be examined when the measuring stud is being pressedagainst the tissue to be examined. Thus, the force applied to press theprobe against the tissue is measured by means of the sensors provided inthe measuring device while the probe is simultaneously emitting anultrasound signal and gathering information about the tissue via thesignal.

In an embodiment of the invention, the measuring device comprisesshifting means for shifting the position of the ultrasound probe betweena projecting outer position against the surface of the tissue to beexamined and an inner position at a distance from the surface of thetissue.

The shifting means for shifting the position of the ultrasound probebetween an outer position and an inner position preferably compriseelements by means of which the ultrasound probe can be moved relative tothe frame of the measuring device e.g. along suitable guide bars.However, another possibility is that the ultrasound probe is fixedlymounted and a movable contact surface or equivalent is provided near theprobe, e.g. around it. By moving the contact surface, the projection ofthe probe can be changed and it can also be brought to a positioncompletely inside the outermost surface defined by the contact surface.

In an embodiment of the invention, the measuring device comprisesscanning means for moving the ultrasound probe inside the measuring armand surveying a given area of the tissue by ultrasound emitted by theultrasound probe. The scanning motion is performed at a known distanceso that the ultrasound echo obtained can be correctly interpreted andthat the results are comparable. This known distance may be a constantdistance or it may be varied provided that it is known exactly as afunction of time or position.

The shifting means and the scanning means are preferably formed by thesame structure, in other words, the means used to retract the ultrasoundprobe to a position inside the measuring device also comprises afunction that makes it possible to use it in this inner position to movethe probe to perform a desired scanning movement. By passing the probeover the tissue, an ultrasound image of the tissue covered by the scancan be produced. The scanning movement itself may consist of a linear,rotating, curved or similar motion, mainly depending on the dimensionsand geometry of the internal frame part and the probe. Anotherpossibility is that the scanning movement is identical with the movementof retraction of the ultrasound probe into the measuring device or itsmovement out of the measuring device.

In an embodiment of the invention, the ultrasound probe used may consistof an ultrasound crystal matrix, i.e. a set of ultrasonic probes thattogether form an area covering all or part of the tissue to be examined.Thus, instead of a scan, the probe matrix can provide information abouta desired area of the tissue.

The method and measuring device of the invention can be successfullyused for quantitative determination of mechanical and structuralproperties of articular cartilage in connection with arthroscopy.Extending acoustic measurements even to the bone under the cartilageallows quantitative determination of the properties of the subchondralbone. It is also very important to know the properties of thesubchondral bone because changes in said properties have been found tobear a relation to degeneration of cartilage.

The method and measuring device of the invention are particularlyadvantageous in measurements of the echo reflected from a cartilage-boneinterface, which provides cartilage thickness and compressioninformation as well as, on the basis of force measurements obtained,material parameters descriptive of the tissue.

After the measurements according to the invention, an actual analysisand diagnosis can be made by combining all measured parameters and theinformation obtained from them. A diagnosis is preferably made bycollecting extensive reference material about all correspondingmeasurements to be performed and entering the new measured data intosuch a file containing reference material, whereupon it is possible toobtain an estimate of the condition of the joint on the basis of thepreviously collected reference material.

The method and measuring device of the invention have significantadvantages as compared with prior art. The invention makes it possibleto achieve measurement of actual material properties of cartilage,measurement of structural and compositional properties of cartilage andsubchondral bone and measurement of cartilage thickness. Moreover,variations in cartilage thickness have no effect on the measurementresults as in prior-art methods.

In the following, the invention will be described in detail withreference to the attached drawings, wherein

FIG. 1 presents a general view of a measuring device according to theinvention,

FIG. 2 presents a magnified sectioned view of the tip of the measuringdevice in FIG. 1 in a first measuring position,

FIG. 3 presents a magnified sectioned view of the tip of the measuringdevice in FIG. 1 in a second measuring position,

FIG. 4 presents a sectioned view of a second embodiment of the structureof the tip of the measuring device,

FIG. 5 illustrates an ultrasound crystal structure used in theinvention, and

FIG. 6 presents a general view of a second measuring device according tothe invention.

The measuring device presented in FIG. 1 comprises an elongated, rigidframe 1, i.e. a handle, and a straight and rigid measuring arm 2attached to the frame. The beveled outer end of the measuring armcomprises a contact surface 3 to be placed against the tissue to beexamined, with a measuring stud 4 protruding from this surface. Inaddition, the measuring arm is provided with at least one sensor,preferably a strain gauge, for measuring the force applied via themeasuring stud from the tissue being examined to the measuring arm. Themeasuring device is connected to suitable means 5 for processing thesignals detected by the measuring arm.

The measuring stud 4 is an ultrasound probe and it is presented ingreater detail in FIGS. 2 and 3. In the first measuring position shownin FIG. 2, the ultrasound probe 4 is in a projecting outer position.Thus, the ultrasound probe can be used to measure mechanical propertiesof a tissue, such as cartilage, by pressing it against the tissue.During the measurement, cartilage thickness and compression are measuredby ultrasound and the force applied to produce the compression ismeasured by one or more strain gauges 6.

In the second measuring position shown in FIG. 3, the ultrasound probe 4has been retracted by means of suitable shifting means 7 into an innerposition, where it is inside the measuring arm at a distance from theoblique contact surface 3 of the measuring arm. In this embodiment,‘shifting means’ refers to an interior part of the measuring arm 2, theultrasound probe 4 being attached to said interior part so that it canbe moved inside the measuring arm in its longitudinal direction. Whenthe oblique contact surface 3 is pressed against the tissue to beexamined, there remains between the tissue surface and the probe 4 a gapof constant size filled with physiologic salt solution. After this, itis possible to perform several pulse-echo measurements on the jointsurface and obtain information as described above about the cartilage,the cartilage-bone interface and the internal parts of the bone.

The ultrasound probe 4 is preferably so mounted and arranged inside themeasuring arm 2 that it can be moved during the measurement at a knowndistance relative to the surface of the tissue to be measured. Thus, bypassing the probe across a given area of the tissue, the tissue areaconcerned or the area under the tissue can be accurately imaged, i.e. anultrasound image of this area can be produced. By maintaining a constantdistance of the probe during all scanning movements over differentsurface areas of the same tissue, measurement results that arecomparable and readily interpretable are obtained. On the other hand, itwill suffice to keep the probe at a known, constant or knowably varyingdistance, because in this case the results can be rendered comparable bycomputational means.

If the ultrasound probe is not moved but only held in a projectingposition as in FIG. 1 or 2, point-specific information about the tissuecan be obtained by pressing the probe 4 into the tissue. Thus, a deviceaccording to the invention may be an embodiment as illustrated in FIGS.1 and 2, wherein the ultrasound probe 4 is fixedly mounted andcontinuously protruding out of the oblique contact surface 3. In thiscase, the tissue to be examined will return a point-specific acousticsignal that provides significant information especially about the boneand the bone interface in the form of possible fading or enhancement ofreflection or scattering.

In the embodiment in FIG. 4, the ultrasound probe 4 is attached toscanning means 8, i.e. to a guide bar along which the probe can be movedback and forth between its extreme positions shown in the figure. Theguide bar 8 extends at a constant distance from the oblique contactsurface 3 at the end of the measuring arm 2. Thus, when a measurement isbeing performed and the contact surface 3 is pressed against the tissueto be examined, the ultrasound probe 4 moves at a constant distance fromthe tissue. The motion of the probe along the guide bar can beimplemented e.g. by mechanical, electric, pneumatic or hydraulic means.

In the embodiment presented in FIG. 5, the ultrasound probe 4 is anultrasound crystal matrix consisting of a plurality of separateultrasound crystals. The matrix covers the tissue area to be examined ata time and the matrix is not moved during the examination. Themeasurement is performed one matrix element or crystal at a time so thatmeasurements by different crystals do not interfere with each other.Thus, point-specific measurements uniformly covering the entire matrixarea of the tissue to be examined are obtained.

In the embodiment presented in FIG. 6, the measuring device is apen-like structure in which a rigid frame 1 and a thinner measuring arm2 extending from it are connected together. The end of the measuring armis provided with an ultrasound probe 4 mounted in an oblique positionrelative to the longitudinal direction of the arm. There is no contactsurface at the end of the measuring arm, but the device is used by onlypressing the probe against the tissue to be examined. Placed in themeasuring arm relatively close to the probe 4 are strain gauges orequivalent sensors 9 for measuring the force applied to press theultrasound probe against the tissue. Thus, the device producespoint-like measurement results for the tissue to be examined.

Thus, when the device of the invention is used to examine tissue such ascartilage and subchondral bone, both mechanical, structural andcompositional properties of the tissue can be determined by the samedevice in connection with arthroscopy.

1. Method for examining a compressible tissue, such as articularcartilage, via arthroscopic indentation measurement, characterized inthat the method comprises pressing the tissue to be examined with anultrasound probe, measuring the thickness and compression of the tissueby means of ultrasound emitted by the probe, and measuring thecompressive force applied.
 2. Method according to claims 1,characterized in that the compressive force applied is measured by meansof a strain gauge.
 3. Method according to claim 1, characterized in thatstiffness moduli, such as Young's modulus and shear modulus, aredetermined for the tissue by a fast measurement.
 4. Method according toclaim 1, characterized in that the reduction of dynamic stiffness, i.e.relaxation speed and indentation stiffness are determined via along-time measurement keeping either the compressive force or thecompression constant.
 5. Method according to claim 4, characterized inthat the indentation stiffness is measured after the lapse of a certainlength of time or in a state of balance.
 6. Method for examining acompressible tissue, such as articular cartilage, via arthroscopicindentation measurement, characterized in that an ultrasound probe isheld at a distance from the tissue surface to be examined so that somephysiologic salt solution remains between the ultrasound probe and thetissue, whereupon pulse-echo measurements on the tissue surface,interior portions of the tissue and/or the bone under it are performed.7. Method according to claim 6, characterized in that, using ultrasoundemitted by the ultrasound probe, a scan across the tissue surface to beexamined is made by moving the ultrasound probe.
 8. Measuring device forexamining a compressible tissue, such as articular cartilage, saiddevice comprising an elongated rigid frame (1), a measuring arm (2)attached to the frame and a measuring stud (4) and means (5) forprocessing signals obtained from the measuring stud, characterized inthat the measuring stud (4) is an ultrasound probe for emittingultrasound into the tissue to be examined and receiving ultrasound fromthe tissue to be examined.
 9. Measuring device according to claim 8,characterized in that the measuring device comprises a contact surface(3) that can be placed against the tissue to be examined.
 10. Measuringdevice according to claim 8, characterized in that the ultrasound probe(4) is an element projecting from the contact surface and the measuringarm comprises a sensor (6) for measuring the force applied to themeasuring arm via the ultrasound probe from the tissue to be examined.11. Measuring device according to claim 10, characterized in that themeasuring device comprises shifting means (7) for shifting the positionof the ultrasound probe between a projecting outer position against thesurface of the tissue to be examined and an inner position at a distancefrom the surface of the tissue.
 12. Measuring device according to claim8, characterized in that the ultrasound probe (4) is placed inside themeasuring arm and attached to scanning means (8) for moving theultrasound probe at a known distance from the tissue to be examined andsurveying a given area of the tissue by ultrasound emitted by theultrasound probe.
 13. Measuring device according to claim 12,characterized in that the known distance is constant.
 14. Measuringdevice according to claim 12, characterized in that the known distanceis variable.
 15. Measuring device according to claim 11, characterizedin that the shifting means are so implemented that they also function asscanning means for moving the ultrasound probe.
 16. Measuring deviceaccording to claim 8, characterized in that the ultrasound probeconsists of an ultrasound crystal matrix that substantially covers thetissue area to be examined.